Field of the Invention
The present invention relates to a see-through head-mounted display, and in particular to a see-through head-mounted display capable of adjusting the virtual image distance with a stable and reliable structure.
Description of the Related Art
In an optical see-through head-mounted display, such as Google Glass, the real image is overlaid with a virtual image. Theoretically, the virtual image distance is required to be substantially the same as the real image distance. If the object being looked at and the virtual image are not located on the same image plane (namely, the distances from the virtual image and from the real image are different to the user's eye), the user's eye has to continuously adjust the focal lengths of their crystalline lens to adapt to the virtual image distance and the real image distance to clearly see both the virtual image and the real image. However, the virtual image distance is fixed according to the optical mechanism of the existing see-through head-mounted display. Using Google Glass as an example: The virtual image distance is fixed to about 3 m, and this design is inconvenient for users because frequent adjustments to the crystalline lens are required in order to allow the user to see the virtual image and the real image, and this can cause the eyes to become tired.
FIG. 1 is a diagram showing the application of a see-through head-mounted display. The left part of FIG. 1 shows that a user wearing a see-through head-mounted display is driving and watching the car in front. Meanwhile, the see-through head-mounted display displays a Google Map. If the see-through head-mounted display doesn't display the virtual image at the distance (such as Z1 shown in FIG. 1) at which the car in front is located, the user has to adjust the crystalline lens of his eye to monitor both the car in front (the real image) and the Google Map (virtual image). The right part of FIG. 1 shows that the user wearing the see-through head-mounted display is reading and focusing his eye on the book at a distance of about 30 cm (such as Z2 shown in FIG. 1). At this moment, a message “You've got Mail” pops up on the see-through head-mounted display. If the distance of the virtual image of this message is still fixed to 3 m, it is apparently very inconvenient for the user to read the message.
FIG. 2 is a diagram showing a structure of a conventional see-through head-mounted display. The structure of the conventional see-through head-mounted display 10 is divided into two portions: an inner optical mechanism 10a and an outer optical mechanism 10b. In FIG. 2, the portion shown in the right side of the dotted line is the inner optical mechanism 10a and the portion shown in the left side of the dotted line is the outer optical mechanism 10b. The optical components of the inner optical mechanism 10a are encapsulated in a nontransparent housing (not shown), and the housing has an opening (not shown) for outputting the image beam. In contrast to the inner optical mechanism 10a, the optical components of the outer optical mechanism 10b are transparent which allow the real image beam (or the environment beam) incident to the user's eye.
The inner optical mechanism 10a includes a LED backlight 101, a polarizer 102, a collimating lens 103, a polarizing beam splitter 104, and a LCOS (liquid crystal on silicon) panel 105. The LED backlight 101, the polarizer 102, the collimating lens 103, and the LCOS panel are arranged at the same side of the polarizing beam splitter 104. First, the LED backlight 101 emits unpolarized light including P-polarized beams of which the polarization is parallel to the incident plane and S-polarized beams of which the polarization is perpendicular to the incident plane. The polarizer 102 passes the light of a specific polarization (S-polarized beam shown in FIG. 2 is an example) and blocks the light of other polarizations. The collimating lens 103 converts the light that travels through the polarizer 102 into parallel light. Those parallel S-polarized beams are incident to the polarizing beam splitter 104. The polarizing beam splitter 104 is a P-type polarizing beam splitter which transmits the P-polarized beam and reflects the S-polarized beam. Therefore, the polarizing beam splitter 104 reflects the incident S-polarized beam to the LCOS panel 105. The LCOS panel 105 has a liquid-crystal layer which can rotate the polarization of the incident light and a reflecting layer which reflects the incident light back to the incident direction. The LCOS panel 105 is driven by voltage to control the polarization of light of each pixel. In this way, the LCOS panel 105 sends image light (including the S-polarized beam and the P-polarized beam) back to the polarizing beam splitter 104. The polarizing beam splitter 104 transmits only the P-polarized beam to the outer optical mechanism 10b from the opening of the inner optical mechanism 10a. 
The outer optical mechanism 10b includes a polarizing beam splitter 106, a quarter wave plate 107, and a concave mirror 108. The quarter wave plate 107, the concave mirror 108, and the user's eye are located at one side of the polarizing beam splitter 106. The real image beam and the virtual image beam are incident to the polarizing beam splitter 106 at the other side of the polarizing beam splitter 106. The polarizing beam splitter 106 is also a P-type polarizing beam splitter, so the P-polarized beam that came from the inner optical mechanism 10a passes through the polarizing beam splitter 106 and is incident to the quarter wave plate 107. The quarter wave plate 107 converts the P-polarized beam into a circularly polarized beam (such as a clockwise circularly polarized beam). The circularly polarized beam is reflected by the concave mirror 108 and is incident to the quarter wave plate 107 again. The quarter wave plate 107 converts the circularly polarized beam into the S-polarized beam. The S-polarized beam is reflected by the polarizing beam splitter 106 to the user's eye. On the other hand, the real image light from the object including the P-polarized beam and the S-polarized beam is also incident to the polarizing beam splitter 106, and only the P-polarized beam of the real image light passes through the polarizing beam splitter 106 and is incident to the user's eye. According to the above structure, the user can see both the real image and the virtual image.
However, the distance d between the LCOS panel 105 and the concave mirror 108 is fixed, so the virtual image distance is also fixed. This causes the aforementioned problem wherein the user has to adjust the crystalline lens frequently to see both the virtual image and the real image. Even though the concave mirror 108 is designed to be driven by a driving mechanism to move to vary the distance from the LCOS panel 105, because the concave mirror 108 and the driving mechanism are exposed outside, the reliability, stability, and durability (such as it being water-proof or dust-proof) of the product are substantially reduced.